1. Field of the Invention
The invention relates to a driving apparatus for an active matrix type luminescent panel which uses capacitive light emitting devices such as organic electroluminescence devices.
2. Description of the Related Background Art
In recent years, with the trend of increasing the size of display devices, thinner display devices have been required, and a variety of thin display devices have been brought into practical use. An electroluminescence display comprising a plurality of organic electroluminescence elements arranged in a matrix has drawn attention as one of the thin display devices.
An organic electroluminescence device (hereinafter, also simply referred to as EL device) can be electrically represented by an equivalent circuit as shown in FIG. 1. As seen from the diagram, the device is replaceable with the circuitry consisting of a capacitive element C and an element E having diode characteristics, coupled to the capacitive element in parallel. The EL device is therefore considered as a capacitive light emitting device. The EL device, when a direct-current emission drive voltage is applied across its electrodes, stores a charge into the capacitive element C. Subsequently, when a barrier voltage or emission threshold voltage specific to this device is exceeded, an electric current starts to flow from the electrode (on the anode side of the diode element E) to an organic functional layer which carries the light emitting layer, so as to emit light in an intensity proportional to this current.
Such known EL-device luminescent panels include simple matrix type luminescent panels and active matrix type luminescent panels. A simple matrix type luminescent panel has EL devices simply arranged in a matrix. An active matrix type luminescent panel has matrix-arranged EL devices each of which is added with a driving device consisting of transistors. A driving apparatus for an active matrix type luminescent panel repeatedly alternates an address period and an emission period to drive each EL device for light emission. In an address period, to-be-operated EL devices on the matrix luminescent panel are designated. In an emission period, an emission voltage is applied to the EL devices designated in the address period.
It is empirically known that the application of voltage to EL devices in a reverse direction not participating in light emission extends the life of the devices. Nevertheless, conventional driving apparatuses for an active matrix type luminescent panel, e.g. as described in Japanese Patent Laid-Open Publication No.Hei 7-111341, apply nothing but a forward voltage to EL devices in an emission period. No reverse bias voltage is applied to the EL devices in either period.
In view of the foregoing, it is an object of the present invention to provide a driving apparatus for an active matrix type luminescent panel, the driving apparatus being capable of applying a reverse bias voltage to each EL device in the active matrix type luminescent panel effectively.
A driving apparatus for an active matrix type luminescent panel according to the present invention is a driving apparatus for an active matrix type luminescent panel including a plurality of capacitive light emitting devices arranged in a matrix, each having polarity, and driving devices for driving the plurality of capacitive light emitting devices individually, the driving apparatus comprising: setting means for setting an address period and an emission period repeatedly on each of the plurality of capacitive light emitting devices in accordance with synchronizing timing in input image data; ON holding means for designating a driving device of the driving devices corresponding to at least a device to be light-emitted of the plurality of capacitive light emitting devices in accordance with the input image data in the address period so that the designated driving device is turned on in the emission period subsequent to the address period; and voltage applying means for applying an emission voltage, in forward polarity, to the device to be light-emitted through the designated driving device in the emission period, wherein the voltage applying means applies a bias voltage, in polarity reverse to the forward polarity, to at least the device to be light-emitted, in the address period.
A driving apparatus for an active matrix type luminescent panel according to the present invention is a driving apparatus for an active matrix type luminescent panel including a plurality of capacitive light emitting devices arranged in a matrix, each having polarity, and driving devices for driving the plurality of capacitive light emitting devices individually, the driving apparatus comprising: setting means for setting an address period and an emission period repeatedly on each of the plurality of capacitive light emitting devices in accordance with synchronizing timing in input image data; designating means for accepting and holding a brightness voltage corresponding to a brightness level in the input image data immediately before the address period, and designating, in the address period, an active device corresponding to at least a device to be light-emitted of the plurality of capacitive light emitting devices in accordance with the brightness voltage; holding means for turning the designated active device on or active in accordance with the brightness voltage in the emission period subsequent to the address period; and voltage applying means for applying an emission voltage, in forward polarity, to the device to be light-emitted through the designated active device in the emission period, wherein the voltage applying means applies a bias voltage, in polarity reverse to the forward polarity, to at least the device to be light-emitted of the plurality of capacitive light emitting devices in the address period.